This invention relates to an electronic control system for internal combustion engines and, more particularly, to an electronic control system which has a plurality of central processing units for independently controlling at least two different engine operations.
A control system for electronically controlling such items as the amount of fuel injection and ignition timing of an internal combustion engine is already known, e.g. from Japanese Provisional Patent Application (Kokai) No. 53-76231, wherein the electronic control system is equipped with a single control processing unit (hereinafter referred to as a "CPU") supplied with input signals indicative of various engine operation parameters such as engine coolant temperature, absolute pressure in the engine intake pipe, a crank angle signal generated whenever the engine crankshaft rotates through a predetermined angular increment, and a reference position signal, two pulses of which are generated per revolution of the crankshaft. On the basis of these input signals, the CPU executes two separate sets of computations, one related to control of fuel injection quantity and the other related to control of ignition timing.
In recent years efforts have been made to meet the demand for improved drivability. This has resulted in more complex operational programs for each of quantities that are to be controlled, such as fuel injection quantity and quantity related to fuel injection timing, and, hence, in a heavier processing load upon the CPU. Consequently, when it is attempted to have a single CPU execute the processing needed to control a plurality of quantities such as fuel injection quantity and fuel injection timing, it is no longer possible to deal with an increase in processing load associated with each quantity. In particular, the demand for engine control of greater precision cannot be met by a single CPU at high engine rotation, where the time available for processing is less than that which can be used when the engine is rotating at low speed. Though this demand can in fact be satisfied if use is made of a CPU having a very high processing capability, such as a 32- or 64-bit CPU, this expedient is impractical because of the high cost involved.
On the other hand, if a plurality of CPU's are employed in order to simultaneously execute the processing needed to control a plurality of quantities such as fuel injection quantity and quantity related to fuel injection timing, each CPU has to be provided with one set of engine parameter sensors, an input circuit, etc. for inputting engine parameter values necessary for the control by the CPU. As a result, as many sets of such sensors, as many input circuits, etc. as the number of CPU's are required, which results in an electronic control system having a very complicated construction, making it difficult to perform assembling and maintenance.
In such an arrangment employing a plurality of CPU's to execute a plurality of controls, if in order for each CPU to execute respective control at proper timing, each CPU is supplied with a cylinder discriminating (CYL) signal, a TDC signal, and a crank angle position signal as aforementioned so that the CPU executes starting and stopping of counting, timing of sampling, etc. by the use of a respective operational program provided exclusively for the CPU, separately from the other CPU's, it requires the use of a set of operational programs for the CPU's, resulting in a further complicated construction of the electronic control system.
Also, in the arrangement employing a plurality of CPU's, a so-called failsafe operation is required in the event of occurrence of abnormality in input and output units associated with each CPU. Desirably, such failsafe operation should be executed based on so-called learnt control data prepared based on operation control data applied when the input and output devices are operating normally and stored in a non-volatile memory device.
However, if each CPU is provided with such a non-volatile memory device to store abnormality-determining data as well as learnt control data, it will result in still further complication of the electonic control system.